It talks about power levels required of upto 200kW, this seems much larger than we can currently produce very easily and I'm not sure how much thrust this would produce. Such an engine is very efficient but I dont see how it is likely to provide enough thrust to propel a structure large enough to service the power requirement necessary for a manned craft.

Anyone got any figures for thrust verses power input.

_________________A journey of a thousand miles begins with a single step.

here's a interesting piece of information from the above wikipedia entry:

Quote:

The ISS VASIMR engine will operate in burst mode. Since ISS's power generation is not great enough, the system will include a trickle-charged battery system allowing for 10 min pulses of thrust. This however, is expected to be sufficient to maintain ISS altitude, eliminating the need for costly, periodic chemical rocket reboosting operations.

Obviously fuel for the VASIMIR will need to be brought up but that is going to be a lot less than is currently needed. I'm surprised that the 5N of thrust the article talks about is enough to stop the ISS orbit from degrading.

There is also a bit about a space tug operating between LEO and the moon. This all seems very hopeful but these systems are very power hungry so I suspect either much more efficient solar cells of possibly a small nuclear power pack will be eeded to take full advantage of it.

_________________A journey of a thousand miles begins with a single step.

atmospheric drag even at altitudes as low as 200km provides a very very small amount of actual thrust, it's just an issue because it's always there and will de-orbit an object over time. 5N is an actually macroscopic amount of force and will easily be able to counteract the effects of drag. you can calculate the drag force fairly easily, and i promise it'll be on the order of 10^-5N or less (probably under 10^-6) even for something the size of the ISS.

you can use any element to fuel vasimr as far as i'm aware, but hydrogen would be a very bad choice in all likelihood because it's so light. i could be wrong on that, it is easy to ionize, but something heavier like argon is definitely better. i doubt the mass benefit of not using argon (really not a big concern, they need extra batteries and stuff even to run the damn thing) is worth it.

Hydrogen is actually quite a good choice for such a plasma thruster. Due to its low mass it leaves the thruster at a high velocity, leading to a high specific impulse (i.e. an efficient use of fuel). Sure, you need a large volume and hence a large tank, but pound-for-pound it gives you the largest delta-v. One way to see this is that in most thrusters, the flow velocity at the exit/throat (hence the specific impulse) will be approx. the sound speed. This scales as ~SQRT(T/M), with T the temperature and M the ion mass. So, the larger the mass, the smaller the specific impulse. (in a magnetic nozzle such as in VASIMR, this might work somewhat differently, but this is the basic idea). One of the challenges of using hydrogen is that it suffers from large volume recombination losses (i.e. ions and electrons find each other fast), due to it being a molecular gas.

The thrust depends largely on the mass flow rate, so that will indeed probably be lower when using hydrogen. However, one of the main advantages of the VASIMR system is that you can - given a total available power - tune it to high thrust or high specific impulse. This probably allows you to run in a mode with still relatively high thrust using hydrogen.

Regarding using different feed gasses: in principle you can indeed ionize and accelerate in this way all gasses (note the word gasses). However, the helicon antenna and the accompanying power supplies that Ad Astra uses are typically tuned to a certain plasma (i.e. gas feed). Take for example the "second stage" ICRH heating in the VASIMR. ICRH stands for Ion Cyclotron Resonance Heating. The name gives away some of the challenges you encounter when trying such things. First of all, you have to hit a certain narrow resonance (by tuning the frequency of the RF power supply). This frequency is the ion cyclotron frequency, which depends on the mass of the ions (and the magnetic field). I don't know the flexibility of Ad Astra's system, but I can image that if you use a very different mass particle, the right frequency can probably not be tuned in with your power supply or effectively coupled into the plasma with the antenna you have. High power RF power supplies that can be tuned over a large frequency range are typically also very expensive. We have a tunable RF power supply in our lab that has an output that is a tiny fraction of the 200 kW that the VX-200 needs and that is already 100k Euro. I think we have another one that is not so tunable with an output power of ~2 kW that is of the same order. So, don't take this lightly. The solid-state power supplies that Ad Astra had ordered turned out to be quite difficult to make (if I read the website correctly) and have delayed the testing schedule by quite a few months. They have been delivered, AFAIK.

To my knowledge (it's been a while since I checked), all their tests of their new VX-200 thruster have been performed with argon.

This, of course, is all very much simplified... Hope I have made this somewhat clearer.

_________________For every complex problem there is a solution that is clear, simple, and wrong. - Henry Louis Mencken

IF the VX-200 prototype is successful, then the "ISS-VASIMR" will be designed to use the waste-hydrogen from the station.

Quote:

The prototype would draw its electrical power from the Station's solar panels. If the test were successful, the VASIMR prototype could be moved to a different part of the Station and used to help keep it in orbit. While ISS orbits the Earth, atmospheric drag gradually pulls it closer to the Earth. Every so often, the Station has to be boosted back into a higher orbit. This is done by a variety of means, but all of them require fuel launched into orbit from Earth. The VASIMR engine, however, could change that, since it would use hydrogen, which is already generated as a waste product on the Station. By using waste hydrogen and electricity from the Station's solar panels, VASIMR could maintain the ISS's orbit without requiring any additional fuel.